Recently, with the development of computers, a data recording device (a peripheral device thereof) is required to satisfy demands for large capacity and for high speed data transfer. To meet the demand, fixed-type magnetic recording devices and fixed-type optical recording devices have been used. In these types of information recording devices, a disc-shaped recording medium is normally adopted, and a rotatable actuator or linear motor is used for driving an information reading section.
Using the above devices, high speed access by a head can be achieved either by making larger the thrust of the head for recording and reproducing information or making a track jump onto a desired recording track accurate. The first method is enabled, for example, by making stronger the magnet used for the linear motor.
In the second method, conventionally the distance to the desired track was measured when accessing information. Recently, improved accuracy is obtained by counting the number of tracks to be jumped to the desired track (track counting method) instead of measuring the distance.
In adopting the track counting method, the following condition must be satisfied: A frequency characteristic of the linear motor system is constant until at a high frequency. In other words, each component of the linear motor is designed such that a resonance thereof is not generated until at a high frequency. By suppressing the resonance, a focus and tracking servo can be made stable, thereby enabling high speed track counting.
The magneto-optical recording medium whereon/wherefrom information can be recorded/reproduced utilizing the magneto-optical effect is a rewritable recording medium. With an increase in the demand for high speed access and data transfer, a so-called overwriting method on the recording medium which does not require an erasing process has been studied and developed. The overwriting method is classified into two types: They are the magnetic field modulation by reversing NS of the magnetic field, and light intensity modulation.
The method for overwriting through light intensity modulation is viewed with interest as the most effective method. For example, in the method disclosed in Laid-open published Japanese patent application No. 63-148446 (148446/1988), only a magnet used in writing is required without requiring an initialization magnet. Moreover, the reversing of the magnetic field is not required in this method. In the optical recording medium of the above arrangement, because the magnetization direction thereof can be reversed only by varying the intensity of the laser beam when writing, a recording can be carried out without an erasing process. Therefore, the above overwriting method enables a substantially improved access speed to be achieved.
However, the recording and reproducing device using the above optical recording medium has the following disadvantage: Since an optical pickup with a weight heavier than the magnetic head is required, and components of the optical pickup and the optical pickup itself tend to have low resonance frequences, access speed is slower than the magnetic recording and reproducing device wherein a magnetic recording medium is adopted. In order to counteract this, a frequency characteristic of the linear motor is set such that the disk resonance is not generated until at a still higher frequency.
As shown in FIG. 5, when adopting a recording medium including a substrate 5 and a recording layer 6 provided thereon, the substrate 5 resonates due to the vibration from a spindle motor (not shown) which rotates the recording medium. For example, FIG. 6 and FIG. 7 show respective frequency characteristics of the linear motor when polycarbonate (PC) and glass are respectively used for the substrate 5 of the recording medium. Here, the frequency characteristic of the linear motor is defined as a frequency response shown by a gain characteristic curve and a phase characteristic curve obtained from a signal A and a signal B. Here, the signal A which is a track error signal is sent under the control of the focus and tracking servo, and the signal B is detected after passing through a servo circuit.
As can be seen from the figures, PC and glass have respective resonance frequencies of 1 kHz and 1.5 kHz. The above resonance frequences are substantially coincident with the respective natural oscillation frequences determined by the characteristics of a material used in the substrate (such as a specific gravity, Young's modules, etc.), and a shape of the substrate. Because the material used in the substrate 5 determines the resonance frequency, it is difficult to avoid the resonance frequency. For this reason, in the conventional recording medium, it is difficult to obtain such a frequency characteristic that the resonance is not generated until at a still higher frequency.